Wear and wear debris are leading causes of implant failure. This has resulted in a very serious interest among surgeons to obtain implants which produce less debris. In particular, ultra high weight polyethylene (UHMWPE) is used extensively in total joint replacement surgery as an articulating surface. This material, however, is prone to wear, producing debris which has been negatively associated with the long term fixation of replacement devices. For instance, Cobalt-Chromium alloy (Co—Cr) heads or condyles, or certain zirconia or alumina heads, articulating against an UHMWPE cup or liner form part of standard ensembles for various total replacement joints, but in recent years it has been reported that polyethylene debris may induce a destructive biologic reaction, osteolysis, which may cause aseptic loosening of the implant and lead to its failure.
More particularly, for example, typical articulating materials for total joint replacements for hip joints include an UHMWPE acetabular cup and a Co—Cr alloy femoral head. Serious clinical problems with this system have been reported to be related to wear of the UHMWPE cup. The harmful effects of polyethylene wear debris have been studied extensively. Biologic reaction to the debris can cause the osteolysis leading to aseptic loosening of the implant. The result is the patient's host bone deteriorates requiring revisional surgery to replace the implant.
The rate of wear of the cup is dependent on the wear mechanism. Least aggressive wear occurs when the prosthesis functions in a stable mode, without third body interactions. In this case, the debris produced is primarily submicron sized. Typical rates of well functioning cups are 0.05˜0.20 mm/year, with estimates of ˜500,000 particles produced per step taken. Rapid wear can occur when third body particles such as dislodged bone cement, dislodged porous coating, or bone chips initiate abrasive wear. These can abrade the UHMWPE directly and roughen the Co—Cr head, both of which increase the wear rate of the polyethylene.
To reduce the problem of such polyethylene wear, ceramic femoral heads have been utilized as a wear couple against the UHMWPE. One major advantage of ceramics over metals is a superior surface. Ceramic materials can be polished to and maintain a smoother surface finish than metal. They also are harder, and thus less likely to incur surface damage, more chemically stable, often more biocompatible, and have a lower friction coefficient when mated with UHMWPE. Studies indicate reductions in wear rate of 30˜400% over the metal-UHMWPE couples. In general, wear studies have indicated that zirconia materials are similar or better than alumina when mated to polyethylene cups.
While use of ceramic femoral heads instead of metal heads is a substantial improvement, a significant amount of polyethylene debris is still generated.
Thus, an articulating wear couple which eliminates the UHMWPE would be desirable.
And so, metal-on-metal and ceramic-on-ceramic wear couples have been proposed.
Ceramic is a notably desirable implant material.
In particular, for total hip replacements, the use of an alumina ball and an alumina cup is known, and it can result in greatly reduced wear rates. Rates of 0.002 to 0.005 mm/year have generally been reported for such well functioning implants, with some reported to be as low as 0.000025 mm/year. However, in some cases, fairly rapid and non uniform wear can occur. These poor results have been attributed to design and installation problems such as implant migration, poor initial alignment, and implant loosening; and to material problems. The enhanced wear can be related to excessive Hertzian contact stresses, which initiate wear, which is followed by a cascade-like wear process as the surfaces roughen and debris is generated. Design and installation problems suggest that the current alumina-alumina combinations are less tolerant of orientation difficulties than implants employing polyethylene cups. Additionally, there are some strength concerns with alumina. Many surgeons are hesitant to use it due to a history of head and cup failures.
In another field, the field of industrial ball valve components, a well established practice is to employ a softer ceramic against a harder ceramic. The harder head wears slightly less than the softer seat. The end result is, with proper selection of materials, the same total wear as a well matched hard couple of the same hardness. It allows the system to “wear in,” reducing stresses in any high contact stress areas after installation.
It is known to employ a TZP zirconia ceramic hip ball in an alumina cup. That softer ball in harder cup couple, however, has its drawbacks. Among these, since that ball is dressed by the cup and gets smaller, it may yield a smaller ball in the cup over time, which may “rattle around.”
Nevertheless, it remains desirable to improve the instant art, noting especially the ever present desire of surgeons and their patients for better and better joint implant performance.